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Swollen leg. Erythema nodosum (red swelling) on the lower leg of a 35-year-old man. Erythema is a redness of the skin due to dilation of the blood vessels which lie beneath its surface. In erythema nodosum, the redness is associated with tender, bruise-like swellings, often on the shins. It commonly occurs after infection with Streptococcus bacteria and subsides with treatment of the underlying condition.